Something is keeping the carrier/lifter from dropping back all the way down to it's normal, lowest position - where it's ready to accept a cartridge from the magazine.
That something could be something as simple as pine needles or other junk in there, or as complex as a worn link tip letting a cartridge slide by & under the lifter.
Either way, it needs to come at least a little bit apart - and unless you have good mechanical ability, take it to a gunsmith.
If you have the ability, remove the large-headed leverpin cover screw fron the middle of the upper left receiver wall, open the lever until the pin near it's front lines up with the holes and drift out the lever pin.
Next, drift out the lower front receiver pin, which holds the front of the lower link in - and the lever/link can be pulled down so you can examine and work on the inside of the loading gate area.
Assemble in reverse order.
Here's a tutorial, w/pics, if you want to try:
http://www.castbullet.com/misc/tdown.htm
